In Japan, heating and cooling systems currently use refrigerants which are substitutes for chlorofluorocarbon, such as HFC-134a (CH2F—CF3), in order to stop serious destruction of the ozone layer due to chlorofluorocarbon. However, since these substitutes for chlorofluorocarbon give greenhouse effect 1300 times greater than that by CO2, which is defined as a greenhouse effect gas, the leakage of the currently used refrigerants to the global environment have a profound effect thereon. Hence, in EU, where hydrofluorocarbon (HFC), perfluorocarbon (PFC), and sulfur hexafluoride (SF6) in the Kyoto Protocol are called “F gases”, the EU car air conditioner refrigerant directive has already banned HFC-134a for use in new-model cars. Likewise, North America is considering future prohibition of this substance (see Non Patent Literatures 1 and 2).
Meanwhile, magnetic refrigeration systems which do not use substitutes for chlorofluorocarbon have attracted attention as heating and cooling systems. Magnetic refrigeration systems, which do not use substitutes for chlorofluorocarbon, are expected to contribute to protection of the ozone layer of the earth's atmosphere and restrain global warming due to greenhouse effect gases.
In a magnetic refrigeration system here, a magnetocaloric effect given by a magnetic material is effectively propagated by a heat exchanging fluid to drive a predetermined refrigeration cycle, thereby providing a range of refrigerant temperature and refrigerating capacity. This is typically called an active magnetic regenerator (AMR) refrigeration method and is widely recognized as an essential method at high temperatures, particularly for magnetic refrigeration at room temperature (see Patent Literatures 1 to 3).
As shown in FIG. 1, a known AMR includes a bed 10 which is, for example, a cylinder filled with a granulated magnetic material 12 and a refrigerant 11 acting as a heat exchanging fluid (water or ethylene glycol) introduced thereinto (the bed is hereinafter referred to as “AMR bed”). Pistons 14a and 14b are provided at both ends of the AMR bed 10 in order to drive the refrigerant 11. These two pistons 14a and 14b move in the direction of the arrow C to make a flow of the refrigerant 11 in the magnetic material 12. One end of the AMR bed 10 is at a low temperature, and the other is at a high temperature.
As shown in FIG. 2, in a typical AMR, a permanent magnet 24, for example, gives a magnetic field to a magnetic material 22 held in an AMR bed 20, and pistons (not shown in the drawing) at both ends of the bed, for example, generate a flow of the heat exchanging fluid.
To be specific, in the initial state, the magnetic material 22 is positioned in the middle of the AMR bed 20 (FIG. 2, (A)). The temperature in the magnetic material 22 is uniform. In the next state, magnetic field generating devices 24 outside the AMR bed 10 apply a magnetic field to the magnetic material 22 in the AMR bed 20 (FIG. 2, (B)). The temperature in the magnetic material 22 is uniform but higher than in the initial state. In the next state, the magnetic material 22 moves in the AMR bed 20 in the direction of the arrow and reaches one end of the AMR bed 20 at a low temperature (FIG. 2(C)). Since heat exchange occurs between a refrigerant 21 and the magnetic material 22, a temperature gradient occurs in the magnetic material 22, so that, in the drawing, the right end is at the lowest temperature and the left end is at the highest temperature.
The magnetic field generating devices 24 are then demagnetized (FIG. 2, (D)). The temperature in the magnetic material 22 uniformly decreases while the temperature gradient generated in the step shown in FIG. 2, (C) is held. Subsequently, the magnetic material 22 moves in the AMR bed 20 in the opposite direction indicated by the arrow by the aid of the resilience given by a spring or the like and reaches the other end of the AMR bed 20 at a high temperature (FIG. 2, (E)). The movement of the magnetic material 22 generates heat exchange between the refrigerant 21 and the magnetic material 22, further increasing the temperature gradient. Repetition of the process from FIG. 2, (B) to FIG. 2, (E) generates such a temperature gradient in the AMR bed 20 that the right end is at the lowest temperature and the left end is at the highest temperature. Providing heat exchangers at both ends produces a refrigerating effect.
When the heat exchanging fluid is caused to flow in the magnetic material 22 in sync with the reciprocating motion of the magnetic material 22 along the axis and the refrigeration cycle is driven in the above-described manner, a temperature difference is generated between both ends of the AMR. A rotation AMR, in which an AMR bed is disposed on part of a circular plate and an AMR cycle is operated using a rotating magnetic material or magnetic field, is known to give an equivalent effect to that given by a reciprocation type.
In another typical AMR shown in FIG. 3, the magnetic material 12 in the AMR bed 10 is driven by a piston 14c in the direction of the arrow D to cause the heat exchanging fluid 11 to flow in the magnetic material. This structure also gives an equivalent effect to that given by the device in FIG. 2.
However, the conventional AMR systems have the following problems:    (i) A mechanism for driving the magnetic material and the heat exchanging fluid is required, which requires high energy input;    (ii) A rotation AMR needs the switching of the high- and low-heat exchanging fluids, causing a heat loss due to the mixture of the fluids during the switching, and    (iii) This complex driving mechanism hinders an increase in the frequency of the refrigeration cycle.